Rare earth (RE) doped optical fibers has found promising applications in the field of optical amplifiers, fiber lasers and sensors. The RE elements doped into the core of such fibers act as the active medium. Different REs like Er, Nd, Yb, Sm, Ho and Tm can be doped to get lasing and amplification covering a wide range of wavelengths. RE doped fiber lasers are replacing gas based or solid state lasers in most of the applications due to their compactness, excellent beam quality and easy handling capability. As a result, there has been around 16% market growth of fiber laser with the overall sales touched $1.35 billion for the year 2012 as reported by Industrial Laser Solutions. Fiber laser devices are suitable for a variety of applications viz. material processing (cutting, grinding and engraving), range finding, medical and military applications. Thus fabrication of RE doped fibers with varied designs, compositions and appropriate RE concentration attracts a lot of research interest. The improvement in the properties of the fibers and increase in the process reproducibility remain the prime objective.
Reference may be made to U.S. Pat. No. 4,826,288 (1989) by R. J. Mansfield, B. C. McCollum, R. P. Tumminelli, “Method for fabricating optical fibers having cores with high rare earth content” wherein, the Modified chemical vapor deposition (MCVD) process with vapor phase chelate delivery technique is adopted for incorporation of high RE ions at the core of the fiber. The core layer deposition was done with silica along with refractive index raising dopant like Al2O3 and RE oxides like Nd2O3 or Yb2O3 and Er2O3. Al2Cl6 and RE(thd)3 served as Al and RE incorporating agent respectively. Helium used as carrier gas of Al and RE compounds. The sources of RE vapor made of glass columns which were filled with solid RE-chelates along with an inert compound such as granulated high purity SiO2 or Al2O3. The columns were heated upto a maximum temperature of 200° C. The temperature of transport line for Nd(thd)3 was in the range of 210° C.-225° C. Various gaseous components were delivered to the reaction zone approximately 250° C., at most. The preferred concentrations of materials in the glass core were: 2-20 wt % of Al2O3, 0.1-4 wt % of Nd2O3 and remainder being SiO2 glass. Another fiber also made with combination of Yb3+ and Er3+ ions. Total RE2O3 concentration was in excess of 5 wt %.
Drawbacks:—They believe to have RE content in the core of preform of about 0.1 to 10 Wt % or more. But in claim part, they only claim about 0.5 wt % of RE2O3. Nothing is said about the length of the preform and distribution of the dopants in the longitudinal as well as the radial direction.
Reference may be made to U.S. Pat. No. 5,961,682 (1999) by Yong-woo Lee, A. N. Guryanov, V. F. Khopin, D. D. Gusovsky, “Method of fabricating optical fiber doped with rare earth element using volatile complex” wherein, reaction of volatile RE-chelate compounds with SiCl4 and O2 took place. The surface of the tube was heated and water cooled to deposit porous core layer on which Al2Cl6 or SiF4 vapors absorbed. Volatile organic metal ligand composed of tris-cyclopentadienyl or tris-isopropylcyclopentadienyl compound of metal ions Er, Dy or Yb used for RE incorporation. Organic ligand bubbler temperature varied in the range of 150-300° C. while Al2Cl6 bubbler temperature was in the range of 140-150° C. Freon gas was used to reduce OH content in the fiber. The difference in the refractive index between cladding layer and core layer greater than 0.025 achieved.
Reference may be made to U.S. Pat. No. 6,474,106 B1 (2002), by C. E. Crossland, Gang Qi, “Rare earth and Alumina-doped optical fiber preform process” wherein, an OVD process has employed to deposit porous soot core layer of SiO2—GeO2—Al2O3—Er2O3 and then cladding layer employed on it as soot-on-soot process and then consolidation of the soot was done following soot-on-glass process in which the mandrel moved leaving a hollow, cylindrical soot blank core. The soot blank core was then consolidated and sintered in certain steps, to form a core rod known as cane. The temperature of solid AlCl3 containing sublimator was varied preferably in between 150° C.-170° C. with Helium/Argon flow rates of about 0.5 to 0.7 slm to incorporate various concentration of Al2O3 in the final preform. Er containing precursors, such as Er(FOD)3 or Er(C30H30F21O6)3, were heated in a bubbler to a temperature range of 130° C.-200° C. Higher Al containing preforms were reported as inclusions free. Er2O3 concentration was around 500 ppm in each preform but concentrations of GeO2 and Al2O3 were varied in between 10 to 20 wt % and 2 to 10 wt % respectively.
Reference may be made to US Patent No. US 2005/0276555 A1 (2005) by T. Haruna, S. Ishikawa, T. Tam, T. Katayama, N. Taira, “Glass-body-producing method and optical glass body and optical fiber” wherein, an organometallic compound is heated from the outside into a glass pipe so that it decomposed into an organic constituent and metallic constituents upstream of the reaction zone. The organic part condensed and deposited there and the metallic part oxidized and deposited with glass layer. The decomposition performed by thermal-decomposition or photo-decomposition by using heat source or light source at temperature 100° C.-1000° C. During consolidation step Cl2 gas was used for dehydration purpose to reduce the OH content. The OH content in the glass body had been reduced to 10 ppm, even at most 1 ppm.
Reference may be made to R. P. Tumminelli, B. C. McCollum, E. Snitzer, Journal of light wave Technology, Vol. 8, No. 11, (1990) pp. 1680-1683, “Fabrication of high concentration rare earth doped optical fibers using chelates” wherein, an individual AlCl3 delivery line and three separate sources of RE-chelates were used. The RE-chelate columns were heated individually to the temperature in between 150 to 210° C. Carrier gas Helium was preheated and passed through RE and Al columns and delivered to a rotating mechanical seal via a heated delivery system. RE, Al and other reactants kept separated to prevent prereaction in the heated delivery tube. A ribbon burner was provided throughout the entire length prior to the reaction zone. The fiber containing 11 wt % Yb2O3 and 0.2 wt % Er2O3 had been prepared. Another fiber containing 1.0 wt % of Nd2O3 had base losses <10 dB/kin at 1130 nm. For high concentration fiber base loss was around 150 dB/km at 1064 nm at 80° C. with OH concentration in between 15 to 20 ppm.
Drawbacks:—Nothing is said about the length of the preform and the distribution of the dopants in the longitudinal as well as the radial direction. For high concentration fibers, background loss and OH concentration is much higher.
Reference may be made to S. D. Jackson, T. Ryan, S. Mossman, Optics Communications, Vol. 216, (2003) pp. 401-404, “High power Tm+3-doped silica fibre laser fabricated using chelate delivery deposition” wherein, a single dopant chamber contained a mixture of Tm3+ and Al3+ chelate which was heated to 200° C. and the vapor is entrained in the flow of O2, helium and other precursor materials. Then oxidation and deposition as porous layer took place which dried using Cl2 gas. The layer then sintered and collapsed in usual manner. The double-clad fiber had a ˜12 μm core diameter with NA of 0.19. Tm3+ concentration was of ˜0.35 wt % and background loss of <10 dB/km at 1300 nm.
Drawbacks:—Concentration level is significantly lower than that already achieved by solution doping method. The chelate heating system was not optimized and the process was limited to be batch type, using only 0.3 gm of chemical. They expect lower background losses but value is not mentioned.
Reference may be made to US 2002/0088252, wherein, a method and apparatus for the manufacture of an optical fiber preform having incorporated therein a rare earth halogen is disclosed.
Drawbacks:—Due to the incorporation of RE-chloride, boat temperature and multi-concentric delivery line temperature has to be maintained >900° C. Moreover, when RE-chloride passes through the innermost part of the concentric tube it encounters higher temperature which may result in decomposition of precursor material.
Reference may be made to E. H. Sekiya, P. Barua, K. Saito, A. J. Ikushima, Journal of Non-Crystalline solids, Vol. 354, (2008) pp. 4737-4742, “Fabrication of Yb-doped silica glass through the modification of MCVD process” wherein, Yb(DPM)3 furnace temperature was varied in the range of 200-250° C., but AlCl3 furnace temperature was kept fixed at 130° C. Temperature of the delivery lines including that of SiCl4 and other gaseous components were kept higher than the temperature of the Yb furnace to avoid condensation of precursor material in the nozzle part. Deposition conditions such as deposition temperature, SiCl4 flow and burner speed was fixed to 1950° C., 0.6 g/min and 145 min/min respectively. Yb3+ concentration obtained for only Yb-doped runs was in the range of 0.15-1.2 wt % while Yb3+ concentration for Yb and Al doped runs was maximum of 0.7 wt % with Al3+ concentration around 0.4 wt %. The variation in refractive index was of ±5% in the longitudinal direction and ±10% in the radial direction.
Drawbacks:—Soot layer deposition took place over a length of 550 mm of silica tube. But uniform core diameter and dopant distribution obtained in a preform of length of only 300 mm. Yb3+ concentration is much lower compared to conventional method. SiCl4 and other gases delivered from normal MCVD gas cabinet also have to send at higher temperature than Yb furnace, otherwise dopants will get condensed in the concentric nozzle part. Variation in dopant distribution in radial direction is around ±10%.
Reference may be made to US 2003/0217569, wherein, a preform for a low fiber optic cable and method and apparatus for fabricating the preform is disclosed. The method includes providing AlCl3 and CVD precursors locally doping CaCl3.
Drawbacks:—Discloses a completely different glass system. The emphasis is on addition of CaO for lowering sintering temperature. Further, RE vapor delivery process is through multi concentric tubes.
Reference may be made to B. Lenardic, M. Kveder, Optical society of America, OSA/OFC/NFOEC 2009, “Advanced vapor-phase doping method using chelate precursor for fabrication of rare earth-doped fibers” wherein, the precursor vapors volatized at temperatures between 100° C.-220° C. and transported to the reaction zone by a system of heated conduits, specially constructed high-temperature rotary seal and sliding precursor vapor injection tube. Instead of burner MCVD is equipped with an induction furnace. Two different designs of sublimator used, bulk sublimator and flat bed sublimator. Flow rate of O2 through SiCl4 bubbler was set to 100 to 250 sccm at bubbler temperature of 35° C. with carriage traversed speed of 100 mm/min. Collapsing was comparatively faster as higher amount of heat supplied by induction furnace. Relationship evaluated between evaporation rate of Yb-chelate and final Yb2O3 concentration in the fiber and evaporation rate of AlCl3 with AlCl3 sublimator temperature. One preform with Er3+ concentration of 2680 ppm and Al3+ concentration of 4900 ppm and another preform with Yb3+ concentration of 31300 ppm and Al3+ concentration of 12000 ppm have been fabricated.
Drawbacks:—Soot layer deposition took place over a length of 600 mm of silica tube. But final preform of length obtained of about 250-350 mm. Larger diameter of substrate tube (30/27 or 25/22) was compulsory to permit sliding injection tube into the substrate tube. Only 20 core layer can be deposited. From the refractive index profiles of the preforms, it is clear that the preforms having high center dip and variation in dopant concentration in radial direction.
Reference may be made to J. Sahu et. al., Optical society of America, OSA/CLEO/QELS 2010, “Rare-earth doped optical fiber fabrication using novel gas phase deposition technique” wherein, the chelate compound was heated in a crucible directly within the MCVD structure which is placed in a non-rotating tube close to the deposition zone. The crucible can be heated upto 800° C. and allowing inert gas to flow down the non-rotating tube and carry the generated vapors to the reaction zone while SiCl4 and other dopants are added to the rotating part of the outer tube. High level of Al incorporated to give NA of 0.24 with base loss ˜3 dB/km. Yb3+ concentration of 9000-20000 ppm-wt was achieved by adjusting crucible temperature with the base loss in the range of 30-70 dB/km. Core diameter of the fabricated fiber was 20 μm (overall fiber diameter 125 μm).
Drawbacks:—As Helium passes through the crucible, it will carry the vapors generated at the upper surface of the crucible. So evaporation rate of RE-chelate compound will be dependent of exposed surface area. It will be problematic to incorporate two or more RE compounds simultaneously.
Reference may be made to U.S. Pat. No. 5,474,588 (1995) by D. Tanaka, A. Wada, T. Sakai, T. Nozawa and R. Yamauchi, “Solution doping of a silica with erbium, aluminium and phosphorus to form an optical fiber” wherein a manufacturing method for Er doped silica is described in which silica glass soot is deposited using VAD apparatus to form a porous soot preform, dipping the said preform into an ethanol solution containing an erbium compound, an Al compound and a phosphoric ester, and desiccating said preform to form Er, Al and P containing soot preform. The desiccation is carried out for a period of 24-240 hours at a temperature of 60 to 70° C. in an atmosphere of nitrogen gas or inert gas. This desiccated soot preform is heated and dehydrated for a period of 2.5-3.5 hours at a temperature of 950 to 1050° C. in an atmosphere of helium gas containing 0.25 to 0.35% chlorine gas and further heated for a period of 3-5 hours at a temperature of 1400 to 1600° C. to render it transparent, thereby forming an erbium doped glass preform. The segregation of AlCl3 in the preform formation process is suppressed due to the presence of phosphorus and as a result the doping concentration of Al3+ can be set to a high level (>3 wt %). It has been also claimed that the dopants concentration and component ratio of Er, Al and P ions having extremely accurate and homogeneous in the radial as well as in longitudinal directions.
Reference may be made to U.S. Pat. No. 6,751,990 (2004), by T. Bandyopadhyay, R. Sen, S. K. Bhadra, K. Dasgupta and M. Ch. Paul, “Process for making rare earth doped optical fiber” wherein, unsintered particulate layer containing GeO2 and P2O5 core layer is deposited and doping by soaking the porous soot layer into an alcoholic/aqueous solution of RE-salts containing co-dopants like AlCl3/Al(NO3)3 in definite proportion is carried out. The porosity of the soot, dipping period, strength of the solution and the proportion of the codopants are controlled to achieve the desired RE3+ concentration in the core and to minimize the core clad boundary defects. In subsequent steps drying, oxidation, dehydration and sintering of the RE containing porous deposit are performed followed by collapsing at a high temperature to produce the preform. The RE3+ distribution in the resulting fiber matches with the Gaussian distribution of the pump beam to increase the overlapping and pump conversion efficiency.
The drawbacks of the above mentioned processes are as follows:    1. Low concentration of dopant material as compared to conventional process;    2. Decomposition and condensation of RE precursor materials occurred prior to reaction zone;    3. Variation of dopant concentration along the longitudinal and radial direction of the preform;    4. Shorter preform length due to loss in effective deposition zone;    5. Process parameters are not optimized.